Random-field models for relaxor ferroelectric behavior (original) (raw)
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Uniaxial relaxor ferroelectrics: The ferroic random-field Ising model materialized at last
Europhysics Letters (EPL), 2002
Owing to their intrinsic charge disorder ferroelectric crystals of strontiumbarium-niobate doped with Ce 3+ materialize the three-dimensional ferroic random-field Ising model (RFIM) as evidenced by order paramenter and susceptibility criticalities with 93 Nb NMR and dielectric spectroscopy, respectively. Upon cooling towards Tc, extreme critical slowingdown due to activated dynamic scaling gives rise to relaxor-like dispersion of the susceptibility and to a metastable ferroelectric nanodomain state with fractal size distribution as imaged by piezoelectric force microscopy.
2015
Canonical and microcanonical Monte Carlo simulations are carried out to study the electrocaloric effect (ECE) in ferroelectrics and relaxor ferroelectrics (RFEs) by direct computation of field-induced temperature variations at the ferroelectric-to-paraelectric phase transition and the nonergodic-to-ergodic state transformation. A lattice-based Hamiltonian is introduced, which includes a thermal energy, a Landau-type term, a dipole-dipole interaction energy, a gradient term representing the domain-wall energy, and an electrostatic energy contribution describing the coupling to external and random fields. The model is first parametrized and studied for the case of BaTiO 3. Then, the ECE in RFEs is investigated, with particular focus on the influence of random fields and domain-wall energies. If the strength or the density of the random fields increases, the ECE peak shifts to a lower temperature but the temperature variation is reduced. On the contrary, if the domain-wall energy increases, the peak shifts to a higher temperature and the ECE becomes stronger. In RFEs, the ECE is maximum at the freezing temperature where the nonergodic-to-ergodic transition takes place. Our results imply that the presence of random fields reduces the entropy variation in an ECE cycle by pinning local polarization.
Local Lattice Dynamics and the Origin of the Relaxor Ferroelectric Behavior
Physical Review Letters, 2008
Relaxor ferroelectricity is observed in many strongly disordered ferroelectric solids. However, the atomistic mechanism of the phenomenon, particularly at high temperatures, is not well understood. In this Letter we show the local lattice dynamics as the origin of relaxor ferroelectricity through the first use of the dynamic pair-density function determined by pulsed neutron inelastic scattering. For a prototypical relaxor ferroelectric, PbMg 1=3 Nb 2=3 O 3 , we demonstrate that the dynamic local polarization sets in around the so-called Burns temperature through the interaction of off-centered Pb ions with soft phonons, and the slowing down of local polarization with decreasing temperature produces the polar nanoregions and the relaxor behavior below room temperature.
Surface Domain Structures and Mesoscopic Phase Transition in Relaxor Ferroelectrics
Advanced Functional Materials, 2011
Relaxor ferroelectrics are a prototypical example of ferroic systems in which interplay between atomic disorder and order parameters gives rise to emergence of unusual properties, including non-exponential relaxations, memory effects, polarization rotations, and broad spectrum of bias-and temperatureinduced phase transitions. Despite more than 40 years of extensive research following the original discovery of ferroelectric relaxors by the Smolensky group, the most basic aspect of these materials -the existence and nature of order parameter -has not been understood thoroughly. Using extensive imaging and spectroscopic studies by variable-temperature and time resolved piezoresponse force microscopy, we fi nd that the observed mesoscopic behavior is consistent with the presence of two effective order parameters describing dynamic and static parts of polarization, respectively. The static component gives rise to rich spatially ordered systems on the ∼ 100 nm length scales, and are only weakly responsive to electric fi eld. The surface of relaxors undergoes a mesoscopic symmetry breaking leading to the freezing of polarization fl uctuations and shift of corresponding transition temperature.
Mapping bias-induced phase stability and random fields in relaxor ferroelectrics
2009
The spatial variability of polarization reversal behavior in the relaxor 0.9Pb͑Mg 1/3 Nb 2/3 O 3 ͒-0.1PbTiO 3 crystal, is revealed on the ϳ100 nm scale using switching spectroscopy piezoresponse force microscopy. Quenched fields conjugate to polarization are found, which show mesoscopic ͑ϳ100-200 nm͒ spatial fluctuations around near-zero bias values. The mapping of the stability gap of the bias-induced phase and conjugate random fields is demonstrated. The origin of the observed nanoscale domains and the field-induced part of the polarization are discussed.
2006
The charge-disordered three-dimensional uniaxial relaxor ferroelectric Sr 0:61 Ba 0:39 Nb 2 O 6 splits up into metastable polar nanoregions and paraelectric interfaces upon cooling from above T c. The frozen polar nanoregions are verified by piezoresponse force microscopy, respond domainlike to dynamic light scattering and dielectric excitation, reveal nonergodicity at T > T c via global aging, and coalesce into polar nanodomains below T c. Contrastingly, the percolating system of unperturbed interfaces becomes ferroelectric with two-dimensional Ising-model-like critical exponents 0, 1=8, and 7=4, as corroborated by ac calorimetry, second harmonic generation, and susceptometry, respectively.
Ising Model Phase Transition Calculation for Ferro-Paramagnetic Lattice
International Letters of Chemistry, Physics and Astronomy, 2013
The position of the phase transition in the two dimensional Ising model were determined byusing Monte Carlo simulation in a quadratic for area of variable length with external magnetic fieldswitched off (B = 0). The magnetization (M) per site (µ), magnetic susceptibility (x) of aferromagnetic and paramagnetic materials were calculated as a function of temperature T for(20×20,40×40,60×60), (80×80,120×120,200×200) spin lattice interactions. Nearest neighborinteraction is assumed (i.e. each spin has 4 neighbors); uses periodic boundary conditions. The Curietemperature (Tc = 2.27 J/kB ) is determined by measuring the magnetic susceptibility at which theferromagnetic and paramagnetic undergoes a phase change from order to disorder. There is thus aphase transition defined by the Curie temperature. The Monte Carlo method were used to check theseresults and to confirm the phase transition. The data are analyzed using the Curie-Weiss law whichcontains the Curie temperature as a parameter.
Relaxor ground state forced by ferroelastic instability in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:mrowmml:msub<mml:mi mathvariant="normal">Kmml:mrowmml:mn0.5mml:msubmml:miBimml:mrowmml:mn0.5...
Physical review, 2020
The complex interplay of local disorder with the structure and dynamics and their role in enhancing the electromechanical response makes relaxor ferroelectric materials fascinating both from the scientific and technological standpoints. It is generally believed that in chemically disordered ferroelectric solid solutions, the normal ferroelectric state gradually yields to a relaxor ground state on increasing the concentration of localized (point-defect-like) random-field centers. That a different kind of process can spontaneously stabilize a relaxor ground state is less known, especially in the family of ferroelectric perovskites. We demonstrate the occurrence of this less-known phenomenon in (1 − x)K 0.5 Bi 0.5 TiO 3-(x)Na 0.5 Bi 0.5 TiO 3 [(1 − x)KBT − (x)NBT]. Unlike the gradual evolution common to most ferroelectric solid solutions, KBT-xNBT exhibits an abrupt crossover from a normal ferroelectric ground state to a full-blown relaxor ground state for x > 0.58. We show that this abrupt crossover to the relaxor state is caused by stabilization of an incommensurate-like/highly disordered M-point ferroelastic distortion in the ergodic/paraelectric temperature regime. The abrupt crossover manifests as composition driven anomalous changes in several important properties such as dielectric response, electromechanical properties, tetragonality, and coercive field, electrostrain, and mimics a scenario often encountered in composition driven interferroelectric transformations.
Loss of relaxor properties due to dipolar interactions in ferroelectrics: A 2D Monte Carlo study
physica status solidi (b), 2012
Phone: The behavior of the dielectric response in relaxor ferroelectric materials is studied by means of Monte Carlo simulations of a simple two-dimensional (2D) generic model. It is showed that the shifting with the frequency of the peak of the temperature dependence of the dielectric permittivity e can be obtained just by considering a randomly distributed pinning anisotropy for the out-plane projection of the polar nanoregions. It is also showed that the consideration of a weak long-range dipolar interaction between nanodomains continuously drives the system to a non-relaxor ferroelectric state in which a single peak is observed for the dielectric permittivity.